Page 105 - Global Tectonics

92 CHAPTER 5

5.1 PLATES AND the time being such structural elements are
ignored.
PLATE MARGINS Within the basic theory of plate tectonics plates are
considered to be internally rigid, and to act as extremely
efficient stress guides. A stress applied to one margin of

a plate is transmitted to its opposite margin with no
The combination of the concept of transform faults
deformation of the plate interior. Deformation, then,
with the hypothesis of sea floor spreading led to the

only takes place at plate margins. This behavior is rather
construction of the theory of plate tectonics. In this
surprising when it is appreciated that plates are typically
theory the lithosphere is divided into an interlocking
only about 100 km thick but may be many thousands of
network of blocks termed plates. The boundaries of
kilometers in width. When plate behavior is examined
plates can take three forms (Isacks et al., 1968).
in more detail, however, it is recognized that there are
1 Ocean ridges (accretive or constructive plate many locations where intra-plate deformation occurs
margins) mark boundaries where plates are (Gordon & Stein, 1992; Gordon, 1998, 2000), especially
diverging. Magma and depleted mantle upwell within the continental crust (Section 2.10.5). Zones of
between the separating plates, giving rise to extension within continental rifts may be many hun-
new oceanic lithosphere. The divergent motion dreds of kilometers wide (Section 7.3). Continental
of the plates is frequently perpendicular to transforms are more complex than oceanic varieties
the strike of the boundary, although this is (Section 8.1). Orogenic belts are characterized by exten-
not always the case and is not a geometric sive thrust faulting, movements along large strike-slip

necessity. In the Pacific it appears to be an fault zones, and extensional deformation that occur
intrinsic characteristic of spreading whenever a deep within continental interiors (Section 10.4.3).
steady direction has been established for some Within oceanic areas there also are regions of crustal
time (Menard & Atwater, 1968). extension and accretion in the backarc basins that are
2 Trenches (destructive plate margins) mark located on the landward sides of many destructive plate
boundaries where two plates are converging margins (Section 9.10).
by the mechanism of the oceanic lithosphere Plates are mechanically decoupled from each other,
of one of the plates being thrust under the although plate margins are in intimate contact. A block
other, eventually to become resorbed into the diagram illustrating schematically the different types of
sub-lithospheric mantle. Since the Earth is not plate boundaries is presented in Fig. 5.1.

expanding significantly (Section 12.3), the rate
of lithospheric destruction at trenches must
be virtually the same as the rate of creation at
ocean ridges. Also included in this category are 5.2 DISTRIBUTION OF
Himalayan-type orogens caused by the collision
of two continental plates (Section 10.1), where EARTHQUAKES
continued compressional deformation may be
occurring. The direction of motion of the
underthrusting plate need not be at right angles to Plate tectonic theory predicts that the majority of the
the trench, that is, oblique subduction can occur. Earth’s tectonic activity takes place at the margins of
3 Transform faults (conservative plate margins) plates. It follows, then, that the location of earthquake

are marked by tangential motions, in which epicenters can be used to define plate boundaries.
adjacent plates in relative motion undergo Figure 5.2 shows the global distribution of the epicen-
neither destruction nor construction. The ters of large magnitude earthquakes for the period
relative motion is usually parallel to the fault. 1961–67 (Barazangi & Dorman, 1969). Although in
There are, however, transform faults that terms of most geologic processes this represents only a
possess a sinuous trace, and on the bends of very short period of observation, the relatively rapid
these faults relatively small regions of extension motions experienced by plates generate very large
and compression are created (Section 8.2). For numbers of earthquakes over a short interval of time.